Electric motors are suitable for high-speed rotation; however, articulated shafts of robots rotate at a lower speed than the electric motors rotate but require high torque. Therefore, an electric motor is applied to an articulated shaft of a robot in combination with a speed reducer in many cases.
In order to maintain accuracy in the rotation direction, a speed reducer for a robot needs to be compact, i.e., small in size, and also needs high rigidity and reduced backlash. Patent Literature 1, which is an example of a conventional technology, discloses a speed reducer with a high reduction ratio that uses mechanical paradox planetary gearing. This is an example of a speed reducer using gearing that satisfies the above-described requirements. In addition, with wave gearing, which is another example of a conventional technology, it is necessary for the speed reducer to be arranged coaxially with the electric motor.